Renewable Energy.

Nanowire Solar Cells
Nanowire Solar CellsEnergy is one of the grand challenges of the present century. It is of great importance to generate electrical power in a renewable way. In order to be accepted by a large audience, it has to be cost-efficient. Solar-energy is one of the few most promising routes to introduce renewable energy at a large scale. At this moment the price per energy unit produced by solar cells is higher than that of electrical energy produced by fossil or nuclear power plants. Practically, the efficiency has to be increased and simultaneously the costs have to be reduced. Nanowires are a promising material system to realize this. Due to the small size of nanowires, different materials can be more easily combined compared to bulk systems, and more sophisticated tandem cells could be fabricated. In addition, light can be more efficiently absorbed by using conical nanowire shapes, and in radial nanowire geometries the optical absorption path length can be disentangled from the charge separation distance allowing more design freedom. This all may enhance the efficiency of solar cells. The cost of NW solar cells may be reduced by using cheaper fabrication methods and by the fact that less of the rare metals are being used in these nanostructured solar cells.

At this moment the record efficiency of III-V tandem solar cells is around 42%. The thermodynamic limit is at 86% for an infinite number of cells in series. We are developing high efficiency solar cells (aiming at 65%) in which the actual harvesting of the solar energy is performed in arrays of III/V semiconductor nanowires. Important aspects are the quality of the tunnel junctions, the surface passivation, and the electrical contacts. We explore the use of plasmonic nanoparticles to focus sunlight into the nanowires. We investigate the embedding of these nanowires into a transparent and flexible polymer for obtaining flexible solar cells. Finally, we employ nanowire solar cells for photo-electrochemical splitting of water. A catalyst or enzyme will be deposited on the nanowires to increase efficiency.

Thermoelectrics
The direct generation of electrical power from heat by thermoelectric effects is a nourished dream for decades, but due to the low efficiency for known materials so far is only exploited for niche applications. The ideal material should simultaneously be a good electrical conductor and poor heat conductor, but these conflicting requirements for an "electron crystal – phonon glass" have not been realized so far. The most promising route towards new thermoelectric materials is a low-dimensional system that strongly scatters phonons but preserves reasonable electrical conduction. Recently, extremely low values have been reported for the thermal conductivity of crystalline semiconductor nanowires, near to, or even lower than, a fundamental lower limit for an amorphous system. These findings promise a breakthrough for applications in thermoelectricity. The thermal conductivity caused by atomic vibrations in nanophononic systems, where the typical dimensions are smaller than the wavelength of phonons is poorly understood and likely requires new concepts for heat transport by collective vibrations.The last few years have witnessed an enormous progress in nanoscale materials science. Several groups, including our group, have shown that it is now possible to accurately control, on the atomic scale, the material composition and structure of nanowires, both in the axial as in the radial direction.We will exploit the full materials control to systematically investigate the thermoelectric effects in nanowires, focused on the most crucial and elusive parameter, the thermal conductivity, which is dominated by atomic vibrations. The goal is to find and understand the ultimate lower bounds on thermal conductivity of nanowires in particular, and of any material in general. These issues will have important implications for the ultimate expectations of thermoelectricity for large-scale applications.